mips_malta: move i8259 initialization after piix4 initialization

i8259 is an ISA device (or at least, depends on the ISA infrastructure to
register its ioport); and the ISA bus is supplied by piix4.  Later patches
make this dependency explicit.

Use qemu_irq_proxy() to stop the cycle by adding an extra layer of
indirection.

Signed-off-by: Avi Kivity <avi@redhat.com>